There are many similarities between adult and adolescent depression. Therefore, it is hypothesized that adolescents with depression would also suffer from disturbance of brain chemical, serotonin. A greater understanding of neurotransmitters involved in adolescent depression is likely to lead to better treatment strategies. We would also conduct a blood test from a specimen obtained during the challenge test to study the gene that regulates the amount of serotonin in a person. Subjects may also be contacted for follow-up. Subjects will be adolescents between the ages of 13-24 yrs who suffer from depression, bipolar depression, and normal controls. Subjects may also suffer from pervasive developmental disorder without mental retardation. Individuals with major medical conditions, pregnant or breast-feeding females, and those individuals addicted to street drugs or alcohol will be excluded. This study will enroll approximately 160 subjects. 80 subjects will be adolescents suffering from major depression the same number of subjects will be matched normal controls. There will be two phases to the study (1) screening and evaluation, (2) serotonin function study. Screening/Evaluation Subjects and their parent/guardian will first be asked to answer questions pertaining to their medical history, females will also be asked to undergo a pregnancy test, complete questionnaires about their current physical and emotional state, behavior patterns, and alcohol/drug use. This evaluation should take four-five hours and will be scheduled as 1-2 sessions. Subjects will also receive a physical examination and battery of laboratory work up. Serotonin function study Subjects will then be scheduled for the serotonin function study. Subjects will be admitted to the hospital one night prior to the experiment. On the morning of the appointment, subjects will be asked to skip breakfast. Only sips of water will be allowed. Subjects will receive an infusion of solution containing either salt water (saline) first or the drug m-chlorophenylpiperazine (m-CPP) through one intravenous line. Both saline and mCPP will be administered to every patient in a blind order (patient and the nurse will not be aware of the order of administration). Subjects will also have about two teaspoons of blood drawn from the intravenous line, several times during the study. The same procedure will be followed after the second infusion. m-CPP can act like serotonin in the brain; the hormone levels in your blood after the m-CPP infusion will allow us to indirectly measure the functioning of serotonin in the brain. We will also be measuring subjects' temperature, pulse and blood pressure several times throughout this period. The blood samples obtained will be used for estimation of prolactin, cortisol, and growth hormone. From one of these samples, we would also determine presence of a gene that regulates amount of serotonin in a person's body and also determine whole blood serotonin level.